This invention relates to a process for the separation of components from mixtures of substances. More particularly, it relates to a process for separating isomeric mixtures.
As industrial chemistry progresses, the demand for purer chemical compounds increases. Distillation and solvent extraction are the two most important processes for the separation of mixtures of substances. Unfortunately, many mixtures of chemical compounds, especially isomeric mixtures, can be separated into the component compounds only with great difficulty and expense or not at all using distillation or solvent extraction.
Distillation has the known disadvantage that the mixtures to be separated must be heated at temperatures which are inversely proportional to the vapor pressure of the substances to be separated. Limits may be set to distillation where the substances to be separated are no longer thermally stable. Use of vacuum widens the field of application of this separation process, but only by an additional boiling interval of about 100.degree. C. to 150.degree. C. The problem is compounded because many isomers have very similar boiling points. Thus, mixtures of isomers are often difficult to separate by distillation. For example, 4,4'-isopropylidenediphenol boils at 195.degree. C. at a pressure of one mm Hg, and 2,4'-isopropylidenediphenol boils at 180.degree. C. at a pressure of one mm Hg. Thus, vacuum distillation at a high temperature and an extremely low pressure could possibly be employed to separate these two isomers, but it is an expensive process, and the required high temperatures promote thermal degradation of the isomeric compounds.
The well-known solvent extraction process is only of limited applicability because the selection of suitable solvents frequently offers great difficulties or because suitable solvents do not exist at all. On the other hand, limits are placed on this process in cases where the solubility characteristics of the components of the mixture to be separated are so similar that efficient separation is no longer obtained. Thus, solvent extraction is not well suited for the separation of many isomers due to the similarity of the isomeric molecules and their characteristics.
Extraction using gases under supercritical conditions has been used to separate many diverse substances. For example, U.S. Pat. Nos. 4,247,570 and 4,251,559 describe the use of supercritical gas extractants in the decaffeination of coffee. Additionally, supercritical gas extraction has been used to remove nicotine from tobacco (U.S. Pat. No. 4,153,063), to treat hydrocarbons, including coal (U.S. Pat. No. 4,028,220) and tar sands (U.S. Pat. No. 4,108,760), and to separate isotopes of chlorine (G. Brit. No. 1,907,924). U.S. Pat. No. 3,969,196 discloses a method of separation which employs a supercritical gas extractant. Said method is limited to a temperature range in which the quantity of organic compound taken up by the gas phase varies inversely with the temperature. U.S. Pat. No. 4,345,976 discloses a process for separating substances of low volatility. Said process is limited in that it requires an entrainer in addition to the supercritical gas. The methods described in the patents cited hereinabove typically operate with separation factors or selectivities of up to about 10.
In view of the deficiencies of the prior art, it would be desirable to have a supercritical gas extraction process for the separation of isomers which did not require an entrainer, and which would be an improved method for separating isomers from isomeric mixtures which are otherwise difficult to separate.